The objectives of the proposed research are to: (1) Continue the characterization of photoreceptor complexes prepared from R. rubrum and R. spheroides. This will involve separation and purification of the major polypeptide components, a determination of their amino acid composition, sequence, and configuration, and exploration of their specific reactivity with bacteriochlorophyll, carotenoid, lipid and each other. A major focus will be understanding the relationship of the phototrap to the antenna pigments. (2) Continue measurements of the photophosphorylation quantum yield in chromatophores under conditions of controlled potential in order to better define cyclic photophosphorylation and its coupling to electron transport in R. rubrum and R. spheroides. (3) Begin membrane reconstitution experiments using purified photoreceptor complexes, phospholipids, and isolated electron transport proteins and phosphorylation coupling factors. (4) Extend the comparison of the photoreceptor complexes and membrane structure to Chromatium bacteria, green algae (Chlorella) and green plants (spinach). (5) Probe the binding site for cytochrome c2 on the purified photoreceptor complex and study the mechanism of electron transport between this component and the primary electron donor unit.